The present invention relates to the treatment of paperboard intended for packaging foodstuffs in order to prevent or inhibit the migration of contaminants from the paperboard into the packaged foodstuffs. Paperboard contaminants even at low concentrations may migrate into foodstuffs packaged in containers made from the paperboard depending upon the solubility and diffusivity characteristics of the paperboard, the foodstuff itself and any barrier coatings applied to the paperboard in the normal course of manufacture. These contaminants which may be perceived either as toxic or as obnoxious because of their health hazard or because they create odor or flavor changes in the food products are objectionable. In particular chlorinated organic compounds such as tetrachlorinated dibenzo-p-dioxin (2,3,7,8-TCDD) and tetrachlorinated dibenzo-p-furan (2,3,7,8-TCDD) which are present in most bleached paperboard products are a cause for concern of the paperboard manufacturer.
It is known in the art of paperboard manufacture to apply coatings to the surface of paperboard for various reasons. For instance, U.S. Pat. No. 4,421,825, discloses the application of a coating comprising titanium dioxide and an acrylic copolymer to paperboard to minimize browning of the paperboard at temperatures up to about 205 degree C. It is also known in the art to apply barriers to the paperboard surface, particularly the food contact surface of the paperboard, to satisfy FDA requirements for safe packaging. These barriers usually take the form of polymer coatings which have achieved FDA clearance for food contact or they may be in the form of a layer of aluminum foil laminated to the paperboard surface. Polyethylene (LDPE, HDPE, LLDPE), ethylene vinyl alcohol copolymers (EVOH), polyvinlyidene chloride (PVDC), nylon, and polyethylene terephthalate (PET) coatings are examples of such barriers. Nevertheless, such barrier materials are not applicable to every packaging situation, particularly where it is desired to keep contaminants such as 2,3,7,8-TCDD and 2,3,7,8-TCDF from the food product, especially at high temperatures.
Studies have shown that at the high temperatures foods may reach in a microwave oven for example, the solubility of 2,3,7,8-TCDD and 2,3,7,8-TCDF is greater in almost every barrier material proposed for use in the food industry today than in paperboard. Moreover, the rate of diffusivity of both 2,3,7,8-TCDD and 2,3,7,8-TCDF has been found to be greater through those materials than through paperboard. Thus the application of conventional barrier materials to paperboard intended for packaging foodstuffs for use in microwave ovens may not be sufficient to attain a contaminant free food product. At cooking temperatures of 225.degree. F. or less, the most up-to-date analytical methods for measuring the presence of 2,3,7,8-TCDD and 2,3,7,8-TCDF has not been able to detect the presence of these contaminants in food. However, at temperatures in excess of 225.degree. F., measurable levels of 2,3,7,8-TCDD and 2,3,7,8-TCDF have been found to migrate from the bleached paperboard into food products. Thus it is presumed that high temperatures are required to motivate these contaminants to migrate from the paperboard to the food product, presumably in the vapor phase. However, for migration within the paperboard, high temperatures are not required.
Accordingly, the present invention was developed to provide a means for reducing and perhaps eliminating the migration of unwanted contaminants from bleached paperboard to food products packaged in containers prepared from the paperboard.